Microwave synthesis of cobalt tungstate for use as stable oxygen evolution catalyst

ABSTRACT

A process of forming an oxygen evolution catalyst includes the steps of: providing Co(NO 3 ) 4 ; providing Na 2 WO 4 ; combining the Co(NO 3 ) 4  and Na 2 WO 4  forming a solution; exposing the solution to a source of microwave energy and initiating a hydrothermal reaction forming hydrated CoWO 4 . The oxygen evolution catalyst including hydrated CoWO 4  may be used to split water into oxygen and hydrogen ions.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

FIELD OF THE INVENTION

The invention relates to a process of forming an oxygen evolution catalyst and apparatus of using cobalt tungstate as a catalyst for the electrochemical and photochemical-electrolysis of water, and in particular to a process and apparatus using hydrated cobalt tungstate as a catalyst for the oxidation of water.

BACKGROUND OF THE INVENTION

Hydrogen has long been considered an ideal fuel source, as it offers a clean, non-polluting alternative to fossil fuels. One source of hydrogen is the splitting of water into hydrogen (H₂) and oxygen (O₂), as depicted in equation (1).

2H₂O→O₂+2H₂   (1)

In an electrochemical half-cell, the water-splitting reaction comprises two half-reactions:

2H₂O→O₂+4H⁺+4e ⁻  (2)

2H⁺+2e ⁻→H₂   (3)

and hydrogen made from water using sunlight prospectively offers an abundant, renewable, clean energy source. However, the oxygen evolution half reaction is much more kinetically limiting than the hydrogen evolution half reaction and therefore can limit the overall production of hydrogen. As such, efforts have been made to search for efficient oxygen evolution reaction (OER) catalysts that can increase the kinetics of OER and increase the production of hydrogen from water. In particular, oxides of ruthenium and iridium have previously been identified. However, as they are among the rarest elements on earth, it is not practical to use these catalysts on a large scale. Therefore, improved OER catalysts would be very useful in the development of hydrogen as an alternative fuel source.

SUMMARY OF THE INVENTION

In one aspect there is disclosed an oxygen evolution catalyst splitting water into oxygen and hydrogen ions that includes hydrated cobalt tungstate.

In another aspect there is disclosed a process for oxidizing water to produce oxygen. The process includes placing water in contact with hydrated cobalt tungstate, the hydrated cobalt tungstate catalyzing the oxidation of water and producing oxygen. The hydrated cobalt tungstate can be a plurality of hydrated cobalt tungstate nanoparticles which may or may not be attached to an electrode with an electrical potential applied between the electrode and the water to generate oxygen.

In a further aspect, there is disclosed a process of forming an oxygen evolution catalyst including the steps of: providing Co(NO₃)₄; providing Na₂WO₄; combining the Co(NO₃)₄ and Na₂WO₄ forming a solution; exposing the solution to a source of microwave energy and initiating a hydrothermal reaction forming hydrated CoWO₄.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a transmission election microscopy (TEM) image of amorphous CoWO₄ nanoparticles;

FIG. 1B is a transmission election microscopy (TEM) image of hydrated CoWO₄ nanoparticles with a microwave dwell time of 1 minute;

FIG. 1C is a transmission election microscopy (TEM) image of crystalline CoWO₄ nanoparticles with a microwave dwell time of 10 minutes;

FIG. 2 is an XRD plot of hydrated CoWO₄ nanoparticles and amorphous CoWO₄ particles;

FIG. 3 is an XRD plot of CoWO₄ nanoparticles synthesized at 200° C. for different microwave dwelling time;

FIG. 4 is a plot of the reaction or dwell time in the microwave and temperature to form amorphous, crystalline and pseudo-crystalline or hydrated phases of CoWO₄;

FIG. 5 is a plot of the surface area and particle size for CoWO₄ as a function of the dwell time in the microwave;

FIG. 6 is a table of the temperature and dwell time to produce hydrated CoWO₄;

FIG. 7A is a graphical representation of cyclic voltammetry traces using a scan rate of 5 mV/s for CoWO₄ for one cycle;

FIG. 7B is a graphical representation of cyclic voltammetry traces using a scan rate of 5 mV/s for CoWO₄ after 20 cycles;

FIG. 8 is a Tafel plot of the of hydrated CoWO₄ (1 min), crystalline (5-60 min) and amorphous CoWO₄;

FIG. 9 is a graphical plot of TGA data for hydrated CoWO₄.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present disclosure provides a method of forming, apparatus and/or catalyst composition for the oxidation of water to generate oxygen gases. The method includes providing a hydrated cobalt tungstate (CoWO₄) catalyst material and adding the catalyst to water under a condition effective to generate oxygen. In one embodiment, the method further includes exposing the water, which contains the catalyst, to light radiation to generate oxygen gases.

A “catalyst” as used herein, means a material that is involved in and increases the rate of a chemical electrolysis reaction (or other electrochemical reaction) and which itself, undergoes reaction as part of the electrolysis, but is largely unconsumed by the reaction itself, and may participate in multiple chemical transformations. A catalytic material of the invention may be consumed in slight quantities during some uses and may be, in many embodiments, regenerated to its original chemical state. The reaction may include a water oxidation or oxygen evolution reaction.

In one aspect a water oxidation catalyst or an oxygen evolution catalyst includes hydrated cobalt tungstate that splits water into oxygen and hydrogen ions.

In a further aspect there is disclosed an electrode for electrochemical water oxidation splitting water into oxygen and hydrogen ions that includes a substrate and an active material in contact with the substrate. The active material includes hydrated cobalt tungstate.

In one aspect, the hydrated cobalt tungstate may be combined with conductive particles such as carbon black and may also include a binder such as NAFION®, a sulfonated tetrafluoroethylene based fluoropolymer copolymer sold by DuPont. The combined material may be attached to an electrode substrate using any method known to those in the art. Various electrode substrates may be utilized that are capable of conducting current such as for example, glassy carbon, carbon black or other materials.

The catalyst can include a plurality of hydrated cobalt tungstate nanoparticles. In some instances, the nanoparticles are uniform in size and can have an average particle size of 1 to 10 nm. In one aspect, the oxygen evolution catalyst including the plurality of hydrated cobalt tungstate nanoparticles may have a surface area of from 40-110 m²/g.

In one embodiment, the hydrated cobalt tungstate is attached to an electrode using any method known to those in the art. For example for illustrative purposes only, absorption techniques, adhesives, deposition techniques and the like can be used to attach the hydrated cobalt molybdenum to the electrode.

In some instances, the electrode can have channels and water can be brought into contact with the catalyst at a rate that allows the water to be incorporated into the electrode channels. In addition, the electrode can be in an aqueous solution and/or be part of an electrochemical cell and/or part of a photo-electrochemical cell, which may or may not include a container.

The container may be any receptacle, such as a carton, can or jar, in which components of an electrochemical device may be held or carried. A container may be fabricated using any known techniques or materials, as will be known to those of ordinary skill in the art. The container may have any shape or size, providing it can contain the components of the electrochemical device. Components of the electrochemical device may be mounted in the container. That is, a component, for example, an electrode, may be associated with the container such that it is immobilized with respect to the container, and in some cases, supported by the container.

In some instances, an electrochemical cell containing an embodiment of the present invention offers a highly efficient method of splitting water using solar illumination, without the need for an applied potential. Upon oxidation of water at a photo-anode, hydrogen protons are generated which are then reduced to form hydrogen gas at a counter electrode. In addition, the oxygen and hydrogen generated from the cell can be passed directly to a fuel cell to generate further power.

In a further embodiment, the electrochemical cell can be driven either by a photo-anode such as a dye sensitized semiconductor or an external potential. The dye sensitized semiconductor acts as a chemical/photo-electrical relay system. For example and for illustrative purposes only, FIG. 1 illustrates a sequence of electron transfer that can occur in a photo-electrical relay system. Examples of such relay systems include ruthenium N-donor dyes such as ruthenium polypyridal dyes that can absorb visible light and accept electrons from a hydrated cobalt molybdenum catalyst material and thereby assist in the oxidation of water that is in contact with the catalyst. In some instances, the photo-sensitizer can be a ruthenium-tris(2,2′-bipyridyl) compound such as ruthenium-tris(2,2′-bipyridyl) chloride.

In another aspect, there is disclosed a process of forming an oxygen evolution catalyst including the steps of: providing Co(NO₃)₄; providing Na₂WO₄; combining the Co(NO₃)₄ and Na₂WO₄ forming a solution; exposing the solution to a source of microwave energy and initiating a hydrothermal reaction forming hydrated CoWO₄. The exposing step may include exposure to microwave energy for various periods of time to elevate the temperature or heat the solution to a desired temperature range. Various time and temperature values are displayed in FIG. 6 to form the hydrated CoWO₄ material.

The exposing step may include exposing the solution to microwave energy from less than one minute to 60 minutes. In one aspect, the exposing step may be from 1 to 10 minutes at a power of 800 Watts. The exposing step may raise the temperature of the solution to a temperature of from 200 to 300 degrees C. Following the exposing step the solution may be cooled and then washed and dried.

The process of forming an oxygen evolution catalyst produces a material that has a pseudo crystalline phase or a hydrated crystalline phase, that provides an increased stability and activity of the catalyst as will be discussed in more detail below. The hydrated phase may be formed at temperatures ranging from 100-240 and dwell times of from less than one minute to 60 minutes as shown in FIG. 4.

The invention is further described by the following examples, which are illustrative of specific modes of practicing the invention and are not intended as limiting the scope of the invention defined in the claims.

EXAMPLES Preparation of Hydrated CoWO₄

Starting materials of Co(NO₃)₄ 6H₂O and Na₂WO₄.2H₂O were purchased from Sigma-Aldrich and used directly without further purification. In a typical synthesis a (0.2M) Na₂WO₄ solution was combined with a (0.2M) Co(NO₃)₂ solution in a stoichiometric manner with strong agitation. The solution mixture was then placed into a glass microwave tube. A microwave assisted hydrothermal synthesis was conducted on a microwave reactor (Anton Paar Microwave 300). The microwave tube was heated to various temperatures at max power (800 W). The exposure to microwaves was maintained for various times as will be discussed in more detail below. Following the exposure to microwaves the tube was cooled by forced air flow. The resulting product was rinsed with DI water multiple times on a centrifuge followed by vacuum drying overnight at 60 degrees C.

A final powder product was examined by TEM as shown in FIGS. 1A-C. It can be seen in the Figures that the hydrated CoWO₄ particles had a much smaller size in comparison to amorphous CoWO₄ particles.

XRD data is shown in FIGS. 3 and 4 for hydrated CoWO₄ with various exposure times to microwave energy and amorphous CoWO₄. The plots for the samples having exposure times of from 0 to 3 minutes display a unique pseudo-crystalline phase with differing height and shifted peaks in comparison to the longer exposed samples. Additionally, the samples having an exposure time less than 5 minutes have an increased surface area, as best seen in FIG. 5.

EXAMPLE II Cyclic Voltammetry (CV) of CoWO₄

To fabricate a working electrode, a catalyst ink was first prepared by sonicating a mixture of catalyst particles, acid-treated carbon black (CB), Na+-exchanged Nafion1 solution with tetrahydofuran, and then drop-casting (10 mL) onto pre-polished glassy carbon disk electrodes (5 mm in diameter). The catalyst film was allowed to dry at room temperature in a sealed container overnight and the final composition of the as expected to be 250, 50 and 50 mg cm22 for CoWO₄, CB and Nafion1, respectively.

The electrochemical measurements were done in a three-electrode glass cell (125 ml) with the working electrode rotating at a rate of 1600 rpm, and Ag/AgCl (3M NaCl) as the reference. The counter electrode (Pt coil) was isolated from the main electrochemical cell using a fritted glass tube. The electrolyte utilized was of 0.4 M Na₂HPO₄ and 0.6 M Na₂SO₄, the pH of which was adjusted using NaOH or HNO₃ solutions. During the electrochemical tests, the electrolyte was continuously purged and masked with ultra-high purity oxygen.

Cyclic voltammogram plots for the CoWO₄ particles after one cycle, and after 20 cycles for amorphous CoWO₄ and various hydrated CoWO₄ samples are shown in FIGS. 7A and B. As shown in the figures, the hydrated CoWO₄ particles after 20 cycles have an increased performance in comparison to amorphous CoWO₄ particles. The hydrated CoWO₄ particles demonstrate an increased stability after repeated CV cycles. In one aspect, the hydrated CoWO₄ maintains 100_percent of its activity after 20 charge cycles.

Tafel plot measurements, as shown in FIG. 8 of hydrated and amorphous CoWO₄ show that hydrated CoWO₄ has better performance per unit of electrode surface area than amorphous CoWO₄ under the same conditions and applied over-potential at steady state. The performance characteristics of the hydrated CoWO₄ indicate an improved electrochemical catalyst for splitting water than may be produced in a large scale using a microwave assisted hydrothermal reaction.

Referring to FIG. 9, there is shown a plot of the TGA analysis of the hydrated CoWO₄ particles. The Total weight loss due to water by TGA was found to be approximately 6.5 to 9 percent with an atomic ratio of CoWO₄x H₂O wherein x is 1.2 to 1.7.

The invention is not restricted to the illustrative examples described above. Examples described are not intended to limit the scope of the invention. Changes therein, other combinations of elements, and other uses will occur to those skilled in the art. The scope of the invention is defined by the scope of the claims. 

Having described our invention, we claim:
 1. An oxygen evolution catalyst splitting water into oxygen and hydrogen ions comprising hydrated Cobalt Tungstate.
 2. The oxygen evolution catalyst of claim 2 wherein the hydrated Cobalt Tungstate has the formula: CoWO₄.
 3. The oxygen evolution catalyst of claim 1 wherein the hydrated Cobalt Tungstate includes a plurality of nanoparticles having a size of from 1 to 10 nm.
 4. The oxygen evolution catalyst of claim 1 further including conductive particles and a binder combined with nanoparticles of hydrated Cobalt Tungstate.
 5. The oxygen evolution catalyst of claim 1 wherein the hydrated Cobalt Tungstate has a CoWO₄ xH₂O wherein x is 1.2 to 1.7.
 6. The oxygen evolution catalyst of claim 1 wherein the catalyst has a surface area of from 40-110 m²/g.
 7. The water oxidation catalyst of claim 1 wherein the catalyst maintains 100% of its activity following 20 charge cycles.
 8. The water oxidation catalyst of claim 1 wherein the hydrated Cobalt Tungstate has a pseudo-crystal structure.
 9. The water oxidation catalyst of claim 1 wherein the hydrated Cobalt Tungstate has a steady state activity greater than amorphous Cobalt Tungstate.
 10. A process for oxidizing water, the process comprising: providing hydrated Cobalt Tungstate; providing water; and placing the water into contact with the hydrated Cobalt Tungstate, the hydrated Cobalt Tungstate catalyzing the oxidation of water.
 11. The process of claim 10, wherein the hydrated Cobalt Tungstate is a plurality of hydrated Cobalt Tungstate nanoparticles.
 12. The process of claim 11, further including applying an electrical potential between the hydrated Cobalt Tungstate and the water.
 13. The process of claim 11, further including adding a photo-sensitizer to the water and exposing the water with photo-sensitizer to electromagnetic radiation, the photo-sensitizer providing an electrical potential between the hydrated Cobalt Tungstate and the water.
 14. The process of claim 13, wherein the photo-sensitizer is a ruthenium-tris(2,2′-bipyridyl) compound.
 15. The process of claim 10, wherein the hydrated Cobalt Tungstate has CoWO₄ x H₂O wherein x is 1.2 to 1.7.
 16. A process of forming an oxygen evolution catalyst including the steps of: providing Co(NO₃)₄; providing Na₂WO₄; combining the Co(NO₃)₄ and Na₂WO₄ forming a solution; exposing the solution to a source of microwave energy and initiating a hydrothermal reaction forming hydrated CoWO₄.
 17. The process of claim 16 wherein stoichiometric amounts of Co(NO₃)₄ and Na₂WO₄ are combined.
 18. The process of claim 16 wherein the exposing step is from 1 to 10 minutes.
 19. The process of claim 16 wherein the exposing step includes elevating the solution to a temperature of from 200 to 300 degrees C.
 20. The process of claim 16 including the step of cooling the solution following the exposing step.
 21. The process of claim 16 further including the step of washing and drying the hydrated CoWO₄ following the hydrothermal reaction. 